自定义 LINQ 中 Distinct 的 Compare，以及扩展 LINQ 的 Distinct，让它支持属性比较

直接贴代码：

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConAppDistinct
{
    class Program
    {
        static void Main(string[] args)
        {
            List<UserInfo> ltUserInfo = UserInfoService.GetAllUserInfo();
            Console.WriteLine("默认调用的 Distinct 无参方法：");
            IEnumerable<UserInfo> query1 = ltUserInfo.Where(u => u.Birthday.Year >= 1987).Distinct();
            foreach (UserInfo item in query1)
            {
                Console.WriteLine(item.Id + "," + item.Name + "," + item.Birthday.ToString("yyyy-MM-dd"));
            }

            Console.WriteLine("=======================================================\n\n");

            Console.WriteLine("调用实现了 IEqualityComparer<UserInfo> 的 Distinct 方法：");
            UserInfoCompare userInfoCompare = new UserInfoCompare();
            IEnumerable<UserInfo> query2 = ltUserInfo.Where(u => u.Birthday.Year >= 1987).Distinct(userInfoCompare);
            foreach (UserInfo item in query2)
            {
                Console.WriteLine(item.Id + "," + item.Name + "," + item.Birthday.ToString("yyyy-MM-dd"));
            }

            Console.WriteLine("=======================================================\n\n");

            Console.WriteLine("调用扩展的 Distinct 方法");
            IEnumerable<UserInfo> query3 = ltUserInfo.Where(u => u.Birthday.Year >= 1987).DistinctBy(u => new{ u.Id,u.Name });
            foreach (UserInfo item in query3)
            {
                Console.WriteLine(item.Id + "," + item.Name + "," + item.Birthday.ToString("yyyy-MM-dd"));
            }
            
        }
    }

    public static class EnumerableExtender
    {
        /* 参考 .NET 源码的实现，利用 .NET Reflector  */
        public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
        {
            HashSet<TKey> seenKeys = new HashSet<TKey>();
            foreach (TSource element in source)
            {
                var elementValue = keySelector(element);
                if (seenKeys.Add(elementValue))
                {
                    yield return element;
                }
            }
        }
    }

    public static class UserInfoService
    {
        public static List<UserInfo> GetAllUserInfo()
        {
            return new List<UserInfo>()
            {
                new UserInfo(){ Id = 1, Name = "张三", Birthday = new DateTime(1985,1,1) },
                new UserInfo(){ Id = 2, Name = "李四", Birthday = new DateTime(1986,2,1) },
                new UserInfo(){ Id = 3, Name = "王五", Birthday = new DateTime(1987,3,1) },
                new UserInfo(){ Id = 4, Name = "赵六", Birthday = new DateTime(1988,4,1) },
                new UserInfo(){ Id = 3, Name = "王五", Birthday = new DateTime(1989,5,1) }
            };
        }
    }

    public class UserInfo
    {
        public int Id
        {
            get;
            set;
        }

        public string Name
        {
            get;
            set;
        }

        public DateTime Birthday
        {
            get;
            set;
        }
    }

    public class UserInfoCompare : IEqualityComparer<UserInfo>
    {
        public bool Equals(UserInfo x, UserInfo y)
        {
            return (x.Id == y.Id && x.Name == y.Name);
        }

        public int GetHashCode(UserInfo obj)
        {
            // return obj.GetHashCode();    // GetHashCode()方法直接用obj.GetHashCode()的话，Distinct不能正常运行。
            return obj.ToString().ToLower().GetHashCode();
        }
    }
}

运行结果如下：

谢谢浏览！